In the course of embryonic development, cells become determined with respect to their type and arrangement. The stability of the determined state has been measured in cultured imaginal disc cells from Drosophila. During cell multiplication in culture, the determined state occasionally changes to yield a different tissue type. Such transitions have been observed and studied by E. Hadorn and his associates who have described the change as "transdeterminations". In the proposed project, the transition of leg tissue to wing tissue will be studied with respect to the factors which increase or decrease the frequency of this transition and which alter the direction of the change, for example, to yield antenna tissue rather than wing tissue when leg tissue transdetermines. We have found that the frequency of the leg to wing transition depends greatly on the developmental stage of the tissue taken for growth in culture. Late third instar tissue transdetermines most frequently, whereas older and younger tissue transdetermine rarely. We will examine further this apparent instability of determined cells at this one stage of development, and will test conditions which cause the expression of suppression of this instability.